The condensation of vapor chemical substances inside the nanometric structures of porous silicon is experimentally and theoretically investigated. The liquid phase covers the pore surface as a thin film and fills a volume fraction in the spongelike structure depending on the physical and chemical properties of each compound. The filling factor of different substances has been measured as a function of the porous silicon film porosity by means of an interferometric technique. We have merged a classical effective medium approximation, such as the Bruggemann theory, with a fractal model of the pore in order to find out how the wetting liquid thickness depends on the porosity. Our results demonstrate that the capillary condensation exhibits a nonlinear behavior at high porosities (greater than 0.8), due to the strong decrease of the vapor confinement degree inside the coalescent nanometric pores.